[logging] Handle trailers-only responses (#33461)

Will be used in the internal logging sink.

src/core/ext/filters/logging/logging_filter.cc

@@ -256,21 +256,19 @@ class CallData {
   void LogServerHeader(bool is_client, CallTracerAnnotationInterface* tracer,
                        const ServerMetadata* metadata) {
     LoggingSink::Entry entry;
-    if (metadata != nullptr) {
-      if (is_client) {
-        if (auto* value = metadata->get_pointer(PeerString())) {
-          peer_ = PeerStringToAddress(*value);
-        }
+    if (is_client) {
+      if (auto* value = metadata->get_pointer(PeerString())) {
+        peer_ = PeerStringToAddress(*value);
       }
     }
     SetCommonEntryFields(&entry, is_client, tracer,
                          LoggingSink::Entry::EventType::kServerHeader);
-    if (metadata != nullptr) {
-      MetadataEncoder encoder(&entry.payload, nullptr,
-                              config_.max_metadata_bytes());
-      metadata->Encode(&encoder);
-      entry.payload_truncated = encoder.truncated();
-    }
+    if (metadata->TransportSize() == 0) return;
+    seen_server_header_ = true;
+    MetadataEncoder encoder(&entry.payload, nullptr,
+                            config_.max_metadata_bytes());
+    metadata->Encode(&encoder);
+    entry.payload_truncated = encoder.truncated();
     g_logging_sink->LogEntry(std::move(entry));
   }
 
@@ -279,6 +277,7 @@ class CallData {
     LoggingSink::Entry entry;
     SetCommonEntryFields(&entry, is_client, tracer,
                          LoggingSink::Entry::EventType::kServerTrailer);
+    entry.is_trailer_only = !seen_server_header_;
     if (metadata != nullptr) {
       MetadataEncoder encoder(&entry.payload, &entry.payload.status_details,
                               config_.max_metadata_bytes());
@@ -340,6 +339,7 @@ class CallData {
   std::string authority_;
   LoggingSink::Entry::Address peer_;
   LoggingSink::Config config_;
+  bool seen_server_header_ = false;
 };
 
 class ClientLoggingFilter final : public ChannelFilter {

src/core/ext/filters/logging/logging_sink.h

@@ -107,6 +107,7 @@ class LoggingSink {
     std::string trace_id;
     std::string span_id;
     bool is_sampled;
+    bool is_trailer_only;
   };
 
   virtual ~LoggingSink() = default;